To increase the efficiency of switch mode converters the secondary side rectification element has gradually been changed from diodes to mosfets (synchronous rectification). This increases the efficiency of the converter and improves the dynamic performance. A mosfet used as a rectification element, where traditionally a diode would be used, is referred to as a synchronous mosfet. The mosfet is turned on when the diode was supposed to conduct. The use of a mosfet as a rectifying element reduces the losses significantly in high current applications.
One drawback, during start-up, is the capability of a mosfet to send currents in both directions. This is a problem if the output of the converter already is biased (a voltage exists on the output) when the converter starts up. If this is not taken into account during startup the output will be pulled to a level dictated by the internal reference, which can damage attached devices. To avoid this, the reference must be set up to a value corresponding to the output, or the start of the converter must be delayed until the reference has reached the correct value. Typical solutions to this problem involve gradual start-up of the synchronous rectifiers by ramping-up the gate voltage to achieve the desired performance. Such a solution is proposed, for example in International Patent Publication No. WO 2007/061369.
The above drawback is addressed, for example, in International Patent Publication No. WO 2006/076642. This document proposes a solution how to start up against an existing voltage below the nominal output on the output of the converter. The duty cycle of the power supply is determined by measuring the input and output voltages and delaying the turn-on until the reference voltage matches the output voltage.
U.S. 2005/0212500 A1 also addresses this problem.